The present invention relates to an information recording method of optically recording information on a recording medium by use of laser light and a recorder using the method.
In a case wherein binary information or information of multivalue more than the binary information is recorded on a recording medium by use of laser light, there is generally required optimization learning for a laser pulse contour, i.e., a so-called recording strategy to record information on the recording medium, which is referred to as recording learning hereinbelow. In the recording learning, a record waveform is reproduced to be evaluated to learn an optimal recording strategy in the system. As evaluation indices, there are employed jitter which is defined as an undesirable shift of a waveform edge along a time-axis, asymmetry which is a shift thereof in an amplitude direction, a “β” value and the like.
On the other hand, in a system which uses laser light to conduct binary information recording on a recording medium and binary information reproduction therefrom, a photoelectric converter to convert laser light into an electric signal as well as an electric signal transmitter and a processing apparatus have a finite frequency characteristic. Hence, as the signal recording and reproduction speed for the recording medium increases, the signal amplitude and the SN ratio of the signal are lowered and inter-symbol interference occurs for signals. Since these appears as distortion in a reproduced waveform, the evaluation indices cannot be correctly detected in the recording learning described above.
As a scheme to prevent these problems, there has been proposed recording learning which is based on an evaluation index and which uses a Partial Response and Maximum Likelihood (PRML) decoder (e.g., JP-A-2005-339690). The PRML is a scheme in which signals reproduced from a recording medium by laser light are equalized by use of schemes such as adaptive equalization to belong to a known Partial Response (PR) class. Maximum Likelihood (ML) decoding is conducted for the signals according to an equalization target of the PR class to estimate a most likely signal sequence to conduct binarization judgment. By conducting the ML decoding after executing the equalization processing according to a PR class similar to the frequency characteristic of the transmission system as above, stable binary signal reproduction is possible even when the band of the transmission system is lower than the reproduced signal band.
Viterbi decoding processing exists as representative decoding processing of the ML decoding. There has been also disclosed adaptive viterbi decoding processing in which a reference value employed in the processing is adjusted to the equalization output waveform of the preceding stage (e.g., JP-A-2004-178627).
The evaluation indices for the reproduced waveform in the recording learning by use of the PRML may include, for example, a difference between the equalization target value and the equalized output waveform, i.e., a so-called equalization error and its total value. However, since the PRML processing uses the adaptive equalization processing, the waveform distortion such as a phase distortion included in the reproduced waveform is corrected in the adaptive equalization processing, and hence the reproduced waveform cannot be correctly evaluated. Therefore, by executing the adaptive equalization processing with a fixed group delay characteristic, waveform distortion information included in the reproduced waveform is sustained. Hence, the reproduced waveform can be correctly evaluated (e.g., WO2005-031743).